This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The membrane of a human red blood cell (RBC) membrane is a composite structure that provides the principal control of both the cell's morphology and mechanics. It consists of a fluid lipid bilayer tethered to a 2D spectrin network, which determines the composite membrane's elasticity. The dynamic properties of this structure influence the ability of RBCs to transport oxygen in circulation. Current techniques for assessing the viscoelasticity of RBCs rely on applying external loads and are limited in obtaining dynamic information over a wide range of frequencies. We apply a non-contact optical interferometric technique to quantify the thermal fluctuations of RBC membranes over a broad range of spatial and temporal frequencies.